Recently, a MOSFET is used widely as a switching element for output in an inverter circuit in a motor driving device. Motor load includes a large inductance component, and noise may be caused by counterelectromotive force of motor when changing phases, or switching element for output may be broken down due to circulating currents of regenerative energy when decelerating or stopping the motor. In particular, when MOSFET is used, since switching speed is fast and parasitic transistor is present owing to its structural characteristics, avalanche tolerance breakdown or latch-up tolerance breakdown may occur. It is attempted to prevent lead-through current from flowing by preventing the MOSFET of upper arm and MOSFET of lower arm of inverter circuit from being turned on simultaneously due to malfunction by noise.
Various measures are proposed for preventing breakdown of MOSFET used as switching element for output in inverter circuit. For example, Japanese Patent Application Laid-Open No. 2003-333883 discloses a method of inserting a pause period when changing phases of inverter circuit. In a certain time after turning off the switching element of the conductive phase, the next switching element is turned on, and the switching elements of both upper arm and lower arm are not turned on simultaneously, so that lead-through current will not flow.
Japanese Patent Application Laid-Open No. 2003-174791 discloses a technology of suppressing generation of noise due to counterelectromotive force of motor when changing phases of inverter circuit. The technology comprises a comparator for detecting counterelectromotive force having a larger voltage value than the supply voltage, and a comparator for detecting counterelectromotive force having a smaller voltage value than the grounding voltage, and when these counterelectromotive forces are detected, the switching element of the corresponding upper arm or lower arm is turned on, and noise is suppressed.
On the other hand, thanks to progress in semiconductor technology, in particular the MOSFET is much reduced in size, there is a trend of decreasing the chip area and decreasing the loss in turn-on time by lowering the on-state resistance from the viewpoint of cost, but at the same time the current density per unit area of element is raised. The structure is also changed from the conventional planar structure to trench structure. In this background, if a wrong element is selected, it easily leads to breakdown, and such risk is increasing.
The prior arts disclosed in these two patent publications are intended to prevent breakdown of MOSFET by adding a special circuit outside of inverter circuit forming the main body of motor driving circuit. As a result, the circuit is complicated, and the cost is increased.